Bonds for solar cell metallization

ABSTRACT

A solar cell can include a substrate and a semiconductor region disposed in or above the substrate. The solar cell can also include a conductive contact disposed on the semiconductor region with the conductive contact including a conductive foil bonded to the semiconductor region.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Some embodiments of the present disclosure allow forincreased solar cell manufacture efficiency by providing novel processesfor fabricating solar cell structures. Some embodiments of the presentdisclosure allow for increased solar cell efficiency by providing novelsolar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 2A illustrate cross-sectional views of a portion of examplesolar cells having conductive contacts formed on emitter regions formedabove a substrate, according to some embodiments.

FIGS. 1B and 2B illustrate cross-sectional views of a portion of examplesolar cells having conductive contacts formed on emitter regions formedin a substrate, according to some embodiments.

FIG. 3 is a flowchart illustrating an example method of forming aconductive contact, according to one embodiment.

FIGS. 4A and 4B illustrate cross-sectional views of forming a conductivecontact in an embodiment with a sputtered first metal region.

FIG. 5 is a flowchart illustrating an example method of forming aconductive contact, according to one embodiment.

FIGS. 6A and 6B illustrate cross-sectional views of forming a conductivecontact in an embodiment with a sputtered first metal region.

FIGS. 7A and 7B illustrate cross-sectional views of forming a conductivecontact in an embodiment with a printed first metal region.

FIG. 8 illustrates cross-sectional views of various example patterningsequences for patterning the foil (and in some instances metalregion(s)) to form a conductive contact.

FIGS. 9 and 10 illustrate cross-sectional views of various examplesequences for selectively applying pressure in forming a conductivecontact.

FIGS. 11A-11C illustrate cross-sectional views of an example sequencefor selectively applying pressure in forming a conductive contact.

FIGS. 12A-12C illustrate cross-sectional views of another examplesequence for selectively applying pressure in forming a conductivecontact.

FIGS. 13-16 illustrate cross-sectional views of various examplesequences for forming a conductive contact in embodiments in which anadditional metal portion is added to the conductive foil.

FIGS. 17A and 17B illustrate cross-sectional views of a solar cellstructure having an intermetallic region.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter of theapplication or uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. §112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” conductive portion of a conductive contact does not necessarilyimply that this conductive portion is the first conductive portion in asequence; instead the term “first” is used to differentiate thisconductive portion from another conductive portion (e.g., a “second”conductive portion).

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based, at least inpart, on those factors. Consider the phrase “determine A based on B.”While B may be a factor that affects the determination of A, such aphrase does not foreclose the determination of A from also being basedon C. In other instances, A may be determined based solely on B.

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

Although many of the examples described herein are back contact solarcells, the techniques and structures apply equally to other (e.g., frontcontact) solar cells as well. Moreover, although much of the disclosureis described in terms of solar cells for ease of understanding, thedisclosed techniques and structures apply equally to other semiconductorstructures (e.g., silicon wafers generally).

Solar cell conductive contacts and methods of forming solar cellconductive contacts are described herein. In the following description,numerous specific details are set forth, such as specific process flowoperations, in order to provide a thorough understanding of embodimentsof the present disclosure. It will be apparent to one skilled in the artthat embodiments of the present disclosure may be practiced withoutthese specific details. In other instances, well-known fabricationtechniques, such as lithography techniques, are not described in detailin order to not unnecessarily obscure embodiments of the presentdisclosure. Furthermore, it is to be understood that the variousembodiments shown in the figures are illustrative representations andare not necessarily drawn to scale.

This specification first describes example solar cells that can includethe disclosed conductive contacts, followed by a more detailedexplanation of various embodiments of conductive contact structures. Thespecification then includes description of example methods for formingthe disclosed conductive contacts. Various examples are providedthroughout.

In a first example solar cell, a conductive foil is used to fabricatecontacts, such as back-side contacts, for a solar cell having emitterregions formed above a substrate of the solar cell. For example, FIG. 1Aillustrates a cross-sectional view of a portion of a solar cell havingconductive contacts formed on emitter regions formed above a substrate,in accordance with an embodiment of the present disclosure.

Referring to FIG. 1A, a portion of solar cell 100A includes patterneddielectric layer 224 disposed above a plurality of n-type dopedpolysilicon regions 220, a plurality of p-type doped polysilicon regions222, and on portions of substrate 200 exposed by trenches 216.Conductive contacts 228 are disposed in a plurality of contact openingsdisposed in dielectric layer 224 and are coupled to the plurality ofn-type doped polysilicon regions 220 and to the plurality of p-typedoped polysilicon regions 222.

In one embodiment, the plurality of n-type doped polysilicon regions 220and the plurality of p-type doped polysilicon regions 222 can provideemitter regions for solar cell 100A. Thus, in an embodiment, conductivecontacts 228 are disposed on the emitter regions. In an embodiment,conductive contacts 228 are back contacts for a back-contact solar celland are situated on a surface of the solar cell opposing a lightreceiving surface (direction provided as 201 in FIG. 1A) of solar cell100A. Furthermore, in one embodiment, the emitter regions are formed ona thin or tunnel dielectric layer 202.

In some embodiments, as shown in FIG. 1A, fabricating a back-contactsolar cell can include forming thin dielectric layer 202 on thesubstrate. In one embodiment, a thin dielectric layer is composed ofsilicon dioxide and has a thickness approximately in the range of 5-50Angstroms. In one embodiment, thin dielectric layer performs as a tunneloxide layer. In an embodiment, the substrate is a bulk monocrystallinesilicon substrate, such as an n-type doped monocrystalline siliconsubstrate. However, in another embodiment, the substrate includes apolycrystalline silicon layer disposed on a global solar cell substrate.

Trenches 216 can be formed between n-type doped polysilicon (oramorphous silicon) regions 220 and p-type doped polysilicon regions 222.Portions of trenches 216 can be texturized to have textured features.Dielectric layer 224 can be formed above the plurality of n-type dopedpolysilicon regions 220, the plurality of p-type doped polysiliconregions 222, and the portions of substrate 200 exposed by trenches 216.In one embodiment, a lower surface of dielectric layer 224 can be formedconformal with the plurality of n-type doped polysilicon regions 220,the plurality of p-type doped polysilicon regions 222, and the exposedportions of substrate 200, while an upper surface of dielectric layer224 is substantially flat. In a specific embodiment, the dielectriclayer 224 is an anti-reflective coating (ARC) layer.

A plurality of contact openings can be formed in dielectric layer 224.The plurality of contact openings can provide exposure to the pluralityof n-type doped polysilicon regions 220 and to the plurality of p-typedoped polysilicon regions 222. In one embodiment, the plurality ofcontact openings is formed by laser ablation. In one embodiment, thecontact openings to the n-type doped polysilicon regions 220 havesubstantially the same height as the contact openings to the p-typedoped polysilicon regions 222.

Forming contacts for the back-contact solar cell can include formingconductive contacts 228 in the plurality of contact openings 226 andcoupled to the plurality of n-type doped polysilicon regions 220 and tothe plurality of p-type doped polysilicon regions 222. Thus, in anembodiment, conductive contacts 228 are formed on or above a surface ofa bulk N-type silicon substrate 200 opposing a light receiving surface201 of the bulk N-type silicon substrate 200. In a specific embodiment,the conductive contacts are formed on regions (222/220) above thesurface of the substrate 200.

Still referring to FIG. 1A, conductive contacts 228 can include aconductive foil 134. In various embodiments, conductive foil can includealuminum, copper, other conductive materials, and/or a combinationthereof. In some embodiments, as shown in FIG. 1A, conductive contacts228 can also include one or more conductive (metal or otherwise)regions, such as regions 130 and 132 in FIG. 1A, between conductive foil134 and a respective semiconductor region. For example, a firstconductive region 130 can include (e.g., aluminum, aluminum/siliconalloy, etc.), which can be printed, or blanket deposited (e.g.,sputtered, evaporated, etc.). In one embodiment, a second conductiveregion can be a bonding region to promote improved bonding between themetal regions, conductive foil, and semiconductor region. Examplebonding regions can include silicon (Si), nickel (Ni), germanium (Ge),lanthanide materials, alloys of aluminum and Si, Ni, Ge, or lanthanidematerials, etc. In various embodiments, the second conductive region canbe deposited on the first conductive region or to the foil before thecell and foil are brought into contact. In one embodiment, as describedherein, an intermetallic phase/zone/region can be formed from at least aportion of the second metal region 132 and conductive foil 134, whichcan allow for a lower temperature and/or less pressure duringthermocompression.

In one embodiment, the conductive foil 134 and the one or moreconductive regions 130 and 132 can be thermally compressed to thesemiconductor region of the solar cell and therefore in electricalcontact with the emitter regions of the solar cell 100A. As describedherein, in some embodiments, as shown in FIGS. 1A and 1B, one or moreconductive regions (e.g., sputtered, evaporated, or printed aluminum,nickel, copper, etc.) may exist between the thermally compressedconductive foil and the emitter regions. Thermally compressed conductivefoil is used herein to refer to the a conductive foil that has beenheated at a temperature at which plastic deformation can occur and towhich mechanically pressure has been applied with sufficient force suchthat the foil can more readily adhere to the emitter regions and/orconductive regions.

In some embodiments, the conductive foil 134 can be aluminum (Al) foil,whether as pure Al or as an alloy (e.g., Al/Silicon (Al/Si) alloy foil).In one embodiment, the conductive foil 134 can also include non-Almetal. Such non-Al metal can be used in combination with or instead ofAl particles. For example, in one embodiment, conductive foil 134 is acopper coated aluminum foil, which can improve solderability and/orwhich can help enable a subsequent plating process. Although much of thedisclosure describes metal foil and metal conductive regions, note thatin some embodiments, non-metal conductive foil (e.g., conductive carbon)and non-metal conductive regions can similarly be used in addition to orinstead of metal foil and metal conductive regions. As described herein,metal foil can include Al, Al-Si alloy, tin, copper, and/or silver,among other examples. In some embodiments, conductive foil can be lessthan 5 microns thick (e.g., less than 1 micron), while in otherembodiments, the foil can be other thicknesses (e.g., 15 microns, 25microns, 37 microns, etc.) In some embodiments, the type of foil (e.g.,aluminum, copper, tin, etc.) can influence the thickness of foil neededto achieve sufficient current transport across the solar cell. Moreover,in embodiments having one or more additional conductive regions 130 and132, the foil can be thinner than in embodiments not having thoseadditional conductive regions.

In various embodiments, the conductive foil 134 may have one or morestrain relief features that can help reduce the risk and amount ofbowing of the wafer. Additional details regarding the strain relieffeatures are described herein.

In various embodiments, conductive regions 130 and 132 can be formedfrom a metal paste (e.g., a paste that includes the metal particles aswell as a binder such that the paste is printable), from a metal powder(e.g., metal particles without a binder, a powder of Al particles, alayer of Al particles and a layer of Cu particles), or from acombination of metal paste and metal powder. In one embodiment usingmetal paste, paste may be applied by printing (e.g., screen printing,ink-jet printing, etc.) paste on the substrate. The paste may include asolvent for ease of delivery of the paste and may also include otherelements, such as binders or glass frit.

In various embodiments, the metal particles of conductive regions 130and 132 can have a thickness of approximately 1-500 microns. Forexample, for an embodiment in which the metal particles are printed, theprinted metal particles can have a thickness of approximately 1-10microns.

In various embodiments, the metal particles can be fired (before and/orafter the conductive foil and conductive regions are thermallycompressed), also referred to as sintering, to coalesce the metalparticles together, which can enhance conductivity and reduce lineresistance thereby improving the performance of the solar cell. Notethat some amount of coalescing of the particles can also occur duringthermal compression. As described herein, the disclosed structures andtechniques can improve the electrical properties of the conductivecontact of a solar cell and/or reduce cost.

Although much of the description describes using thermocompressiontechniques and structures (including conductive foil) instead of platedmetal, in some embodiments, additional metal can be plated to conductivefoil 130. For example, nickel and/or copper can be plated according toan electroless or electrolytic plating technique. Note that in oneembodiment, zinc may be added, for example in a Zincate process, toenable plating on aluminum. Various examples of embodiments that canhelp facilitate plating are described herein.

Turning now to FIG. 1B, a cross-sectional view of a portion of anexample solar cell having conductive contacts formed on emitter regionsformed in a substrate is illustrated, according to one embodiment. Forexample, in this second exemplary cell, thermally compressed conductivefoil can be used to fabricate contacts, such as back-side contacts, fora solar cell having emitter regions formed in a substrate of the solarcell.

As shown in FIG. 1B, a portion of solar cell 100B includes patterneddielectric layer 124 disposed above a plurality of n-type dopeddiffusion regions 120, a plurality of p-type doped diffusion regions122, and on portions of substrate 100, such as a bulk crystallinesilicon substrate. Conductive contacts 128 are disposed in a pluralityof contact openings disposed in dielectric layer 124 and are coupled tothe plurality of n-type doped diffusion regions 120 and to the pluralityof p-type doped diffusion regions 122. In an embodiment, diffusionregions 120 and 122 are formed by doping regions of a silicon substratewith n-type dopants and p-type dopants, respectively. Furthermore, theplurality of n-type doped diffusion regions 120 and the plurality ofp-type doped diffusion regions 122 can, in one embodiment, provideemitter regions for solar cell 100B. Thus, in an embodiment, conductivecontacts 128 are disposed on the emitter regions. In an embodiment,conductive contacts 128 are back contacts for a back-contact solar celland are situated on a surface of the solar cell opposing a lightreceiving surface, such as opposing a texturized light receiving surface101, as depicted in FIG. 1B.

In one embodiment, referring again to FIG. 1B and similar to that ofFIG. 1A, conductive contacts 128 can include a conductive foil 134 andin some embodiments, one or more additional conductive regions, such asconductive regions 130 and 132. The conductive foil 134, and the one ormore conductive regions can be thermally compressed to the semiconductorregion of the solar cell and/or to one or more conductive regionsbetween the foil and the semiconductor region and therefore inelectrical contact with the emitter regions of the solar cell 100A. Theconductive contact description of FIG. 1A applies equally to theconductive contact of FIG. 1B but is not repeated for clarity ofdescription.

Turning now to FIG. 2A, the illustrated solar cell includes the samefeatures as the solar cell of FIG. 1A except that the example solar cellof FIG. 2A does not include the one or more additional conductiveregions (regions 130 and 132 of FIG. 1A). Instead, conductive foil 134is thermally compressed and bonded directly to the semiconductor regionof the solar cell.

Similarly, the illustrated solar cell of FIG. 2B includes the samefeatures as the solar cell of FIG. 1B except that the example solar cellof FIG. 2B does not include the one or more additional conductiveregions (regions 130 and 132 of FIG. 1B). Instead, conductive foil 134is thermally compressed directly to the semiconductor region of thesolar cell.

Although certain materials are described herein, some materials may bereadily substituted with others with other such embodiments remainingwithin the spirit and scope of embodiments of the present disclosure.For example, in an embodiment, a different material substrate, such as agroup III-V material substrate, can be used instead of a siliconsubstrate.

Note that, in various embodiments, the formed contacts need not beformed directly on a bulk substrate, as was described in FIGS. 1B and2B. For example, in one embodiment, conductive contacts such as thosedescribed above are formed on semiconducting regions formed above (e.g.,on a back side of) as bulk substrate, as was described for FIGS. 1A and2A.

Using thermally compressed particles as a contact can reduce contactresistance and improve conductivity of the conductive contact and as aresult, improve the performance of the solar cell. Furthermore, thedeformed particles can increase the cohesion of the particles andadhesion of the particles to the solar cell. Moreover, in an embodimentin which Al particles are used (whether in a first or second conductiveregion or as part of the conductive foil), deforming the Al particlescan break the oxide shell around the Al particles further enhancing thedeformed Al particles' conductivity. When particles are deformed, theparticle to particle contact area increases, thus aiding interdiffusionof atoms during sintering, which ultimately can improve the conductivityand cohesion of the particles.

Turning now to FIG. 3, a flow chart illustrating a method for forming aconductive contact is shown, according to some embodiments. In variousembodiments, the method of FIG. 3 may include additional (or fewer)blocks than illustrated. For example, in some embodiments, welding thefoil, as shown at block 306, may not be performed.

As shown at 302, a conductive foil can be placed over a semiconductorregion. As described herein, the conductive foil can be aluminum foil,copper foil, copper-coated aluminum foil, among other examples. Althoughdescribed as foil, note that the foil does not necessarily have to havethe shape of a sheet, but can be pre-patterned in the shape of wires,ribbons, fingers, etc. FIG. 4A illustrates an example cross-sectionalview conductive foil 404 being placed over wafer 400. Note that in someembodiments, conductive foil 404 can overlap wafer 400 completely, incontrast to the illustration of FIG. 4A. Further note that for ease ofillustration, the actual semiconductor regions are not illustrated inFIG. 4A but the semiconductor regions of FIGS. 2A or 2B apply equally tothe illustration of FIG. 4A. As shown in FIG. 4A, conductive foil 404can be positioned over dielectric regions 402 a, 402 b, 402 c, and 402d.

In one embodiment, the conductive foil can be vacuumed or otherwise heldtightly to the surface of the wafer to help ensure a proper fit up ofthe foil to the wafer. For example, the surface upon which the wafersits (e.g., plate 410 in FIG. 4B) can include a number of holes throughwhich air is removed from the foil/wafer interface.

At 304, a thermocompression technique can be applied to the conductivefoil. The thermocompression technique can include heating and applyingmechanical force to the foil. Heating the foil can reduce the amount ofmechanical force needed by reducing the yield strength of the foil andhelp improve the bond. In some embodiments, heating the conductive foilcan include heating the foil at temperatures above 200 degrees Celsiusand applying mechanical force can include applying pressure of at least1 psi.

In one embodiment, mechanical force can include vertical compressionand/or lateral mechanical force. Mechanical force can be applied by atool 412, such as a roller, plate, squeegee, among other tools. Toolscan be made from graphite, have a graphite coating, or be made from orhave a coating from another material such as Marinite A or Marinite C orother such that the tool will not adhere to the foil duringthermocompression. As one example of a tool, in one embodiment, theconductive foil can be pressed between two substantially parallel platesthat are subjected to pressure to force the wafer and foil together. Inanother embodiment, the foil and wafer can be pressed together usingflexible membranes with pressurized fluid on the other side of themembrane. The pressurized fluid pressing can be dry-bag or wet-bagisostatic pressing, which can be cold or hot isostatic pressing.

In various embodiments, the heating and mechanical force can be appliedtogether or the heating can occur first, followed by application ofmechanical force. For example, in one embodiment, wafer 400 can beplaced on a hot surface 410, such as a hotplate set at approximately400-500 degrees Celsius as shown in FIG. 4B. In one embodiment, tool 412can be heated, in addition to or instead of heating the surface on whichthe wafer is positioned. Note that approximately 400-500 degrees is anexample used for an aluminum-based foil in the range of 20-40 micronsbut other examples exist. For example, for a foil of a different typeand thickness, a different temperature and time sufficient to soften thefoil can be used. Moreover, as described herein, by using a secondconductive region from which an intermetallic phase can be formed, alower temperature and less force can be used for thermocompression.

In one embodiment, in addition to or instead of forming athermocompression bond, ultrasonic agitation of the foil-seed interfacecan be used, which can result in a stronger bond or reduce theprocessing temperature required for bonding.

In one embodiment, the temperature used to heat the conductive foiland/or conductive region(s) can be similar to a temperature used in aformed gas anneal (“FGA”) process for fabrication of a solar cell.Accordingly, in one embodiment, heating the foil and/or applyingmechanical force can be performed during the FGA process in the sametool, which can result in fewer pieces of fabrication equipment and savetime during the manufacturing process.

In some embodiments, the tool, plate(s), and/or surface can be made of amaterial that is less likely to form a bond with the conductive foil orwafer. For example, in an embodiment in which the conductive foilincludes aluminum, the tool may include graphite to make the tool itselfless likely to stick or bond to the foil.

In some embodiments, the conductive foil can expand more than the wafersuch that, upon cooling, the wafer/foil assembly can bow as the foilcontracts more than the wafer. To help alleviate wafer bowing, one ormore strain relief features can be added to the foil. For example, inone embodiment, the mechanical force can be selectively applied (e.g.,unevenly distributed). Selective force can be applied with a patternedtool, which in some embodiments can be a patterned platen or pattereddie. By patterning the pressure field applied to the foil, regions ofthe foil can be alternately strongly and weakly bonded. Weakly bondedareas can then be more easily removed from the cell by etching, laserablation, tearing, a combination of etching, ablation, and/or tearing,etc.

In some embodiments, the foil can be bonded to the wafer at discretepoints or lines (strongly bonded) with the foil bending away from theplane of the wafer at other points (weakly bonded). The weakly bondedfoil regions can delaminate from the wafer allowing for plasticdeformation (contracting) of those regions, which can relieve strainfrom the system and reduce wafer bowing. Various examples of the strainrelief features and patterned die can be seen at FIGS. 9, 10, 11A-11C,and 12A-C. As one specific example, as shown in FIGS. 11A-11C, force canbe applied in a pattern that approximates the finger pattern of thesolar cell, which can not only create strain relief features but canalso concentrate the mechanical force in the regions corresponding tothe contacts such that the overall mechanical force applied to the cellas a whole can be lessened. Other strain relief features can includeperforations or slits in the foil, among other examples.

In some embodiments, additional conductive material can be added on topof the conductive foil. For example, an additional conductive foil canbe added either before or after the heating and mechanical force isapplied. As one specific example, a region of copper (e.g., a copperpaste, foil, or powder) can be added on the conductive foil andthermally compressed together with the conductive foil and substratesuch that the outermost portion of the contact is copper, which can makesoldering more effective, for example, to interconnect multiple solarcells. In another embodiment, the additional conductive material caninclude plated metal, such as copper.

At 306, the conductive foil can be welded in some embodiments. In otherembodiments, the welding at block 306 may not be performed. In oneembodiment, the conductive foil can be welded to the semiconductorregions of the solar cell without performing the thermocompressiontechnique at block 304. In another embodiment, the conductive foil canbe welded to the semiconductor regions after performing block 304.Welding can be laser welding, friction welding, or other types ofwelding and can be applied as spot welding (e.g., in locationscorresponding to fingers) and can increase the strength of the bonds.

As shown at 308, the conductive foil can be patterned. Numerouspatterning examples exist and are described in more detail below. Forexample, the patterning at 308 can include mask and etch patterning,groove and etch patterning, mask, groove, and etch patterning, amongother examples. In an embodiment in which a thermocompression bond issubstantially continuous over a large area of the wafer, wet etching canbe used as a method of patterning with a reduced risk of trapping etchchemistry in the structure.

Turning now to FIG. 5, a flow chart illustrating a method for forming aconductive contact is shown, according to some embodiments. In variousembodiments, the method of FIG. 5 may include additional (or fewer)blocks than illustrated. For example, in some embodiments, welding thefoil, as shown at block 508, may not be performed. Or, in someembodiments, applying the thermocompression technique at 506 may not beperformed. Moreover, in various embodiments, the description of themethod of FIG. 3 applies equally to the description of the method ofFIG. 5. Accordingly, for clarity of explanation, some description ofthat description is not repeated.

As shown at 502, a first metal region (or non-metal conductive region)can be formed over a semiconductor region. In one embodiment, the firstmetal region can be a paste, particles, or a thin continuous layer,etc., and can be formed in a variety of manners, such as sputtering,printing metal (e.g., printed in a pattern, such as a finger pattern),evaporating, otherwise depositing, etc. The first metal region caninclude metal, solvents, binders, viscosity modifiers, etc. Metalexamples include aluminum, aluminum-silicon, other aluminum alloys,among other examples.

In various embodiments, the thickness of the first metal region can beless than 5 microns, and in one embodiment, can be less than 1 micron.

In one embodiment, one or more additional regions can be formed over thefirst metal region (e.g., by sputtering, printing, evaporating, etc.).In one embodiment, the additional region(s) (e.g., a second metal ornon-metal conductive region) can include a bonding region that can forman intermetallic phase or alloy with the conductive foil. Examples ofsuch a bonding region include nickel, germanium, silicon, lanthanidemetals, or an alloy of at least aluminum with one of those materials. Inone embodiment, the bonding region can form the intermetallic phase at atemperature below the melting point of the conductive foil (e.g., for anembodiment in which the conductive foil is an aluminum foil, below themelting point of aluminum). By forming the intermetallic phase or alloy,a sufficient bond can be formed between the foil, intermetallic phase,first metal region, and substrate even with a lower thermocompressiontemperature and/or less pressure. Accordingly by using less pressure anda lower temperature, the risk of damage to the wafer can be reduced.Moreover, the intermetallic phase can enable bypassing thethermocompression technique altogether and provide for a sufficient bondby another technique, such as welding. Accordingly, in one embodiment,the foil and second metal region can be welded and, in doing, so form anintermetallic phase.

In one embodiment, the first and second metal regions can be formed withthe same tool (e.g., PVD, tool, CVD tool, etc.) and therefore addsminimal processing time and cost to the manufacturing process.

In an embodiment in which the additional metal region(s) is a printedseed, the first metal region can be fired to volatize solvents and anyviscosity modifier as well as to activate any binders. After firing, theparticles of the paste can be bonded to each other and to the substrate.Firing can be performed before or after thermocompression.

As shown at 504, a conductive foil can be placed over the first metalregion, and in embodiments in which additional metal region(s) are overthe first metal region, over the additional metal region(s). Asdescribed herein, the conductive foil can be aluminum foil, copper foil,copper-coated aluminum foil, among other examples. Moreover, asdescribed herein, multiple layers of conductive foil can be used (e.g.,a layer of aluminum foil, and layer of copper foil, etc.).

In one embodiment, a thermocompression technique can be applied to theconductive foil as shown at 506, and as described at block 304 of FIG.3.

At 508, in one embodiment, the conductive foil can be welded asdescribed at block 306 of FIG. 3.

As shown at 510, the foil and any metal regions (e.g., first, second,etc.) can be patterned as described at block 308 of FIG. 3 and asdescribed herein.

FIGS. 6A and 6B illustrate cross-sectional views of forming a conductivecontact in an embodiment with a sputtered first metal region. As shownin FIG. 6A, conductive foil 604 is placed over first metal region 606,which is formed over wafer 600. As shown, first metal region issputtered such that at least a portion of it is between dielectricregions 602 a-d and at least a portion of it is on those dielectricregions. Note that in some embodiments, conductive foil 604 can overlapwafer 600 completely. Further note that for ease of illustration, theactual semiconductor regions are not illustrated in FIG. 6A but thesemiconductor regions of FIG. 1A or 1B apply equally to the illustrationof FIG. 6A.

As shown in FIG. 6B, wafer 600 is placed on a hot surface, such ashotplate 610 set at approximately 400-500 degrees Celsius. Pressure isthen applied by tool 612 to apply the compression portion of thethermocompression technique. Other example thermocompression techniquesexist as well, as described herein. For example, tool 612 may apply boththe heat and pressure instead of, or in addition to, the hotplateapplying heat.

FIGS. 7A and 7B illustrate cross-sectional views of forming a conductivecontact in an embodiment with a printed seed first metal region. Incontrast to FIG. 6A, the first metal region 706 in FIG. 7A is a printedseed region that does not cover the entire surface of wafer 700. Forexample, the pattern in which the seed region is printed can be in afinger pattern so that the first metal region 706 does not necessarilyneed to be patterned. Although the dielectric regions of FIGS. 6A and 6Bare not illustrated in FIG. 7A for clarity, those regions maynevertheless be present in the example of FIG. 7A as well. Note that inFIGS. 7A and 7B, the particles of the first metal region 706 have beenspecifically illustrated but the skilled person will understand that thestructure of the first metal region 706 is similar to the structure ofthe other metal regions described herein.

FIG. 7B illustrates applying thermocompression to the first metal region706 and conductive foil 704. As shown, the particles of the first metalregion 706 can be deformed, which can cause the particles to adhere toone another and to the substrate better and decrease line resistance,thereby enhancing conductivity and solar cell performance. As is thecase with the example of FIGS. 6A-6B and 7A-7B, other examples to heatand apply pressure exist. For example, instead of or in addition tousing a hot plate 710, the tool 712 can be heated. Or, the tool andassembly can be placed in an oven (e.g., as part of an FGA process orotherwise).

FIG. 8 illustrates cross-sectional views of various example patterningsequences for patterning the foil (and in some instances metalregion(s)) to form a conductive contact. Although FIG. 8 illustratesthree patterning sequences, others can exist. The first three views canapply to all three sequences. As shown, a first metal region 806 isformed over wafer 800 and a conductive foil is placed over the firstmetal region 806. As described herein, note that the first metal region806 may not be present in some embodiments (e.g., as shown in FIGS. 2A,2B, and 3). And note that in some embodiments, one or more additionalmetal regions can be present. Similar patterning techniques can applyregardless of whether a metal region or regions is present between theconductive foil and semiconductor region.

The leftmost patterning sequence illustrates a mask, groove, and etchsequence. As shown, a non-patterned mask 816 (e.g., non-patterned etchresist, film, etc.) is applied on conductive foil 804, for example,across substantially the entire surface of the conductive foil. Mask 816is then patterned as shown in the next view in the sequence, whether bylaser ablation, mechanical grooving, or otherwise. In one embodiment,the conductive foil can also be patterned or grooved, for example bylaser ablation. Next, a chemical etch is applied and the mask isstripped with the resulting cell with conductive contacts illustrated inthe final sequence.

The middle patterning sequence is similar to the mask, groove, and etchsequence except that mask 816 is applied (e.g., printed) in a particularpattern instead of as a blanket mask. As shown, a chemical etch isapplied and then mask 816 is stripped with the resulting cell withconductive contacts illustrated in the final view of the sequence.

The rightmost sequence illustrates a groove and etch patterningtechniques which does not include applying a mask (blanket, patterned,or otherwise). As shown, the actual conductive foil 804 can be groovedin locations corresponding to the dielectric regions 802 (and whereseparation between fingers is intended). In one embodiment, lasergrooving those locations can remove a majority of the thickness in thoselocations. Accordingly, the groove does not entirely cut through theentire foil, instead leaving a portion. A chemical etch is then appliedwhich removes the remaining portion from the groove thereby separatingthe foil (and any metal regions between the foil and semiconductorregion) into the pattern.

Similar to the groove and etch example, in one embodiment, theconductive foil 804 can include an etch-resistant coating on its outersurface. The etch-resistant coating can be patterned with a laser orotherwise, followed by one or more chemical etches.

The disclosed thermocompression structures can offer many advantageswhen considering the various patterning techniques. For example, becausethe conductive foil (and if applicable, conductive region(s)) iscompressed substantially uniformly across the solar cell, wet etchingcan be used with a reduced risk of trapping etch chemistry in the solarcell as opposed to techniques in which the foil or metal regions leavegaps in which the etch chemistry could be trapped.

FIGS. 9 and 10 illustrate cross-sectional views of various examplesequences for selectively applying pressure in forming a conductivecontact. The examples of FIGS. 9 and 10 illustrate first metal region906 but note that in other embodiments, it can be omitted. Selectivepressure is applied, as shown by the downward arrows, by a patterned die912/1012 or other tool to selectively apply pressure to bond selectregions of the foil, conductive region(s), and semiconductor region. Forexample, the select regions can correspond to what ultimately are theconductive contacts or solar cell fingers. As illustrated, the selectivepressure can result in one or more strain relief features 918/1018 thatcan help alleviate the tendency for the wafer to bow, as describedherein. Note that the strain relief features 918/1018 may not be bondedto the first conductive region, as illustrated, or it can be weaklybonded as compared to the other regions.

Moreover, in embodiments in which the semiconductor regions areseparated by a trench structure, applying selective pressure can reducethe risk that edges of the semiconductor regions break off and thereforereduce the risk that unwanted unpassivated portions of the semiconductorregion exist (which could exist if enough of the edge breaks off).

Similar to the patterning techniques illustrated in FIG. 8, variousexample patterning techniques are illustrated in FIGS. 9 and 10. Variousdetails described at FIG. 8 will not be repeated in the description ofFIGS. 9 and 10. In one embodiment, the patterning technique shown on theleft-hand side of FIG. 9 illustrates application of a patterned mask916, followed by a chemical etch (which removes the portion of the foiland first metal region not covered by mask 916), and stripping of themask 916.

The patterning technique shown on the right-hand side of FIG. 9illustrates a groove and etch technique similar to the groove and etchtechnique of FIG. 8. As shown, the regions of foil between respectivesemiconductor areas are grooved (e.g., by laser ablation). Althoughshown as a complete groove of the regions of foil, in some embodiments,grooving can remove less than all of the thickness of the foil at thegrooved location. Or, in some embodiments, all of the thickness of thefoil at the grooved location can be removed and a portion of the firstmetal region can also be removed. The final view in the right-hand sideof FIG. 9 illustrates the resulting solar cell after applying a chemicaletch. When compared with mask and etch example of FIG. 8, theillustrated groove and etch example can result in wider fingers and canalso result in at least a portion of the strain relief feature remainingin the cell. Note, however, that a wider groove area can be used inother examples such that the width of the fingers in a groove and etchexample can be substantially similar to that of the mask and etchexample.

FIG. 10 illustrates a similar main sequence as in FIG. 9 but with twodifferent patterning techniques than in FIG. 9. The left-hand patterningtechnique of FIG. 10 illustrates a non-patterned mask technique, similarto the non-patterned mask technique of FIG. 8, whereas the right-handpatterning technique of FIG. 10 illustrates a mask, groove, and etchtechnique similar to that of FIG. 8.

In the various examples of FIGS. 8-10, the non-patterned mask, groove,and etch technique can enable the use of alternate choices of mask(e.g., a PET sheet, which cannot be printed) and can result in lessetching of the finger top surface thereby leaving more metal intact onthe fingers. Additionally, the mask, groove, and etch technique canpermit less metal loading of the etch bath.

FIGS. 11A-11C illustrate cross-sectional views of an example sequencefor selectively applying pressure in forming a conductive contact. Theexamples of FIG. 11A-11C illustrate applying selective pressure toconductive foil 1100 to achieve a finger pattern in which the metal ofthe conductive foil is substantially parallel to the corresponding metalin the first metal region 1102. Such a pattern is referred to herein asa fine M2 pattern.

FIGS. 12A-12C illustrate cross-sectional views of another examplesequence for selectively applying pressure in forming a conductivecontact. The examples of FIGS. 12A-12C illustrate applying selectivepressure to conductive foil 1200 to achieve a finger pattern in whichthe metal of the conductive foil is substantially perpendicular to thecorresponding metal in the first metal region 1202. Such a pattern isreferred to herein as a coarse M2 pattern. As shown, to achieve such apattern, a high temperature dielectric 1206 can be formed over firstconductive regions 1202 corresponding to an opposite metal type fromwhich the foil finger 1200 is. For example, for regions in which thep-type fingers of the conductive foil overlap the n-type first metalregions, the dielectric 1206 can be applied to those n-type first metalregions. And for areas where n-type fingers of the conductive foiloverlap p-type first metal regions, the dielectric 1206 can be appliedto those p-type first metal regions.

Although FIGS. 11A-11C and 12A-12C illustrate the concepts of fine andcoarse M2 patterns using selective pressure application, in otherembodiments, substantially uniformly distributed pressure applicationcan be used to achieve the illustrated coarse and fine M2 patterns.

FIGS. 13-16 illustrate cross-sectional views of various examplesequences for forming a conductive contact in embodiments in which anadditional metal portion is added to the conductive foil.

FIG. 13 illustrates a conductive foil 1304 with an additional conductivelayer 1316 on top. In various embodiments, the additional conductivelayer 1316 can be a sputtered or evaporated layer, it can be a coatingof the conductive foil, or it can be a separate foil that is thermallycompressed during thermal compression of the foil or welded in someembodiments.

In one embodiment, the conductive foil 1304 is an aluminum foil with anadditional conductive layer 1316 of copper. As shown, a plating mask1312 can be applied in a particular pattern on the additional conductivelayer 1316. Copper 1308 and/or tin 1310 can be plated to the additionalconductive layer 1316. An etchback process can then be applied byremoving the plating mask 1302 and etching the metal layers 1316, 1304,and 1306 in a suitable etchant or series of etchants resulting in theremoval of the mask, and various metal layers resulting in the patternedsolar cell in the final view of FIG. 13.

By using a copper surface as the additional conductive layer, platingwith copper and/or soldering to cell-to-cell interconnects can beperformed.

FIG. 14 illustrates a conductive foil 1404 with an additional conductivelayer 1416 as in the example of FIG. 13. Different than FIG. 13, theexample in FIG. 14 does not include an additional metal region (as infirst metal region 1306 in FIG. 13). Instead, the conductive foil 1404can make direct contact with the silicon between openings in dielectricregions 1402. The remaining views of FIG. 14 are similar to those ofFIG. 13; therefore, the description applies equally to the views of FIG.14.

FIG. 15 illustrates a conductive foil 1504 with additional conductiveregions 1516 as bond pads, such as solderable bond pads. Variousexamples of additional conductive regions include copper foil, nickelfoil, among other solderable materials. As shown, additional conductiveregions 1516 exist at the bond pads, as shown in FIG. 15. The bond padscan be thermocompressed to the conductive foil 1504, or it can be weldedor otherwise bonded to conductive foil 1504. The patterning sequenceillustrated in the left-hand side of FIG. 15 illustrates a mask and etchpatterning technique whereas the right-hand side of FIG. 15 illustratesa groove and etch patterning technique, both of which are describedherein.

FIG. 16 illustrates a process in which a copper mask is applied to thecopper coated foil. As shown, a copper etch mask 1602 can be applied toadditional copper layer 1616, followed by an etching of portions ofcopper (e.g., by ferric chloride, peroxide/HC1, or other copper etchant,etc.) followwed by an etching of portions of the remaining conductivefoil, resulting in the structure in the final view of FIG. 16.

In one embodiment, the coating on the foil can be patterned (e.g., laserpatterned) to form a copper etch mask, which can then be used as a hardetch resist mask for KOH etching of aluminum.

FIGS. 17A and 17B illustrate cross-section views of a solar cellstructure having an intermetallic region. FIGS. 17A and 17B illustrate asolar cell structure similar to the structures of FIGS. 1A and 1B havingfirst and second metal regions 1706 and 1708 and a conductive foil 1704.As shown, both structures have an intermetallic zone/phase/region 1710,as described herein, which is formed from the second metal region 1706and the conductive foil 1704. The difference between FIGS. 17A and 17Bis that FIG. 17A illustrates thermally compressed conductive foil andfirst and second metal regions to form the intermetallic phase 1710whereas FIG. 17B illustrates using heat (e.g., laser heat) to locallymelt the metal to form the intermetallic phase 1710.

In one embodiment, the second metal region 1708 can be nickel,germanium, or silicon. In various embodiments, the thickness of thefirst metal region 1706 can be less than 5 microns (e.g., less than 1micron), and the conductive foil 1704 can be between 10 and 100 microns(e.g., 30-60 microns).

In one embodiment, the thickness of the second metal region 1708 ischosen such that the bond strength of the resulting intermetallic jointis high and that the volume of the conductive foil converted into theintermetallic phase is limited so as not to reduce its mechanicalstrength or its electrical conductivity.

For the example of FIG. 17A, the intermetallic zone formation can be atleast partially dependent on the thickness of the second conductiveregion. For the example of FIG. 17B, the volume that is affected by theintermetallic formation can be dependent on the thickness of theconductive foil and the lateral dimension of the melt zone 1715. Thus,in some embodiments, the thickness of the second conductive region andthe lateral dimension of the melt zone 1715 can be used to define theratio of concentration of the two metals in the intermetallic mixture toa value that is suited to produce a mechanically stable joint.

By using a second metal region that allows for the creation of anintermetallic zone, a lower temperature and/or less pressure can beused, thereby reducing the risk to damage to the solar cell.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

1. A solar cell, comprising: a substrate; a semiconductor regiondisposed in or above the substrate; and a conductive foil thermallycompressed to the semiconductor region.
 2. The solar cell of claim 1,further comprising a first metal between the conductive foil and thesemiconductor region.
 3. The solar cell of claim 2, further comprising asecond metal between the conductive foil and the first metal. 4-6.(canceled)
 7. The solar cell of claim 1, further comprising a contactregion for coupling the solar cell to another solar cell, wherein thecontact region includes a different type of metal than the conductivefoil.
 8. A method of fabricating a solar cell, the method comprising:placing a conductive foil over a semiconductor region disposed in orabove a substrate; heating the conductive foil; applying mechanicalforce to the heated conductive foil to form a conductive bond for thesolar cell.
 9. (canceled)
 10. The method of claim 8, wherein saidapplying mechanical force is performed using a patterned die.
 11. Themethod of claim 8, further comprising forming a strain relief feature.12. The method of claim 8, further comprising forming a solderableregion on the conductive foil, wherein the solderable region includes amaterial other than that of the conductive foil.
 13. The method of claim8, wherein said heating the foil and said applying mechanical force areperformed as part of a forming gas anneal process.
 14. The method ofclaim 8, wherein said applying mechanical force is performedsubstantially uniformly across the conductive foil corresponding to thesubstrate.
 15. The method of claim 8, further comprising after saidapplying mechanical force, laser welding the conductive bond.
 16. Themethod of claim 8, further comprising grooving and etching regions ofthe foil to form contact fingers for the solar cell.
 17. A method offabricating a solar cell, the method comprising: forming a first metalregion over a semiconductor region disposed in or above a substrate;placing a conductive foil over the first metal region; bonding theconductive foil to the first metal region; and patterning the conductivefoil and the first metal region.
 18. The method of claim 17, furthercomprising forming a second metal region over the first metal region,wherein said bonding includes bonding the conductive foil to the firstand second metal, and wherein said patterning includes patterning theconductive foil and the first and second metal regions
 19. The method ofclaim 18, wherein said bonding includes forming an intermetalliccompound from the conductive foil and the second metal region.
 20. Themethod of claim 17, further comprising applying an ultrasonic agitationtechnique to the conductive foil and first metal region.
 21. The methodof claim 17, wherein said patterning the conductive foil includes:printing a mask over regions of the conductive foil; and applying achemical etch.
 22. The method of claim 17, wherein said patterning theconductive foil includes: laser grooving regions of the conductive foilto remove a majority of a thickness of the regions; and applying achemical etch.
 23. The method of claim 17, wherein said patterning theconductive foil includes: applying a non-patterned mask across theconductive foil; patterning the non-patterned mask; and applying achemical etch.
 24. The method of claim 17, wherein said patterning theconductive foil includes: laser patterning an etch resistant coating ofthe conductive foil; and applying a chemical etch.